This invention pertains generally to the field of semiconductor diode lasers and particularly to edge emitting distributed feedback semiconductor lasers.
Semiconductor diode lasers are formed of multiple layers of semiconductor materials. The typical semiconductor diode laser includes an n-type layer, a p-type layer and an undoped active layer between them such that when the diode is forward biased electrons and holes recombine in the active region layer with the resulting emission of light. The layers adjacent to the active layer typically have a lower index of refraction than the active layer and form cladding layers that confine the emitted light to the active layer and sometimes to adjacent layers. Semiconductor lasers may be constructed to be either edge emitting or surface emitting. In an edge emitting Fabry-Perot type semiconductor laser, crystal facet mirrors are located at opposite edges of the multi-layer structure to provide reflection of the emitted light back and forth in a longitudinal direction, generally in the plane of the layers, to provide lasing action and emission of laser light from one of the facets. Another type of device, which may be designed to be either edge emitting or surface emitting, utilizes distributed feedback structures rather then conventional facets or mirrors, providing feedback for lasing as a result of backward Bragg scattering from periodic variations of the refractive index or the gain or both of the semiconductor laser structure.
Semiconductor lasers having CW power in the watt-range and narrow bandwidth, e.g., less than 2 xc3x85 full width half maximum (FWHM), would be desirable for a variety of applications. Examples include 0.894 xcexcm diode lasers which may be used for polarizing Cs to generate spin-polarized Xe gas for magnetic resonance imaging, low-chirp pump sources for solid state lasers, and in spectroscopy sources for monitoring environmental gases. Conventional broad stripe (xe2x89xa725 xcexcm) semiconductor lasers used for obtaining high powers typically have a spectral width of about 20 xc3x85 FWHM or more at high drive levels and broaden further under quasi-CW operation. Significant improvements in spectral width can be obtained using distributed feedback (DFB) gratings or distributed Bragg reflectors (DBR) rather than Fabry-Perot mirror facets for optical feedback. 278 mW CW power with about 1 xc3x85 of wavelength variation, resulting from mode hopping, has been reported for narrow-stripe DBR lasers. J. S. Major, et al., Electron. Lett. Vol. 29, No. 24, p. 2121, 1993. Using DFB phase-locked laser arrays, narrow bandwidth operation has been obtained from large apertures at relatively long wavelengths (xcex=1.3 xcexcm to 1.5 xcexcm). 120 mW pulsed operation has been reported from a 45 xcexcm aperture device (xcex=1.3 xcexcm), Y. Twu, et al., Electron. Lett. Vol. 24, No. 12, p. 1144, 1988, and 85 mW CW from a 72 xcexcm aperture device (xcex=1.55 xcexcm), K. Y. Liou, et al., Tech. Dig. 13th IEEE Int. Semicond. Laser Conf., Paper D7, 1992. For applications where (lateral) spatial coherence is not necessary, a broad-stripe laser with a DFB grating is apparently well suited for achieving high CW powers with narrow spectral linewidth.
A limitation is encountered with DFB lasers designed to operate at shorter wavelengths including visible light wavelengths, in that conventional diode lasers grown on GaAs substrates, which can emit in the range of wavelengths between about 0.6 xcexcm to 1.1 xcexcm, generally have optical confinement layers containing aluminum as well as cladding layers containing aluminum. Due to the high reactivity of aluminum (i.e., essentially instant oxidation when exposed to air), it has proven to be very difficult to make single frequency lasers of the DFB type in the foregoing wavelength range in which the grating is buried within the multi-layer semiconductor structure. Consequently, the commercially available high power, narrow linewidth lasers have been of the distributed Bragg reflector (DBR) type, in which the grating is outside of the active lasing part of the structure. However, such DBR devices suffer from the major drawback of mode hopping that occurs with increasing drive current due to changes in the lasing-region index of refraction with increasing drive power.
The present invention encompasses a high power edge emitting semiconductor laser with very narrow spectral width that can be tailored to operate at precisely selected wavelengths including wavelengths in the visible range. In accordance with the invention, typical CW powers in the watt range are obtainable with a narrow linewidth of 2 xc3x85 FWHM or less. Consequently, such lasers are well suited to applications requiring precise narrow linewidth laser sources, such as for polarizing cesium or rubidium for use in magnetic resonance imaging with spin polarized xenon.
The edge emitting semiconductor laser of the invention includes a substrate and an epitaxial structure preferably grown on orientation on the substrate. The epitaxial structure includes a layer with an active region at which light emission occurs, upper and lower confinement layers adjacent the active region layer, upper and lower cladding layers adjacent the confinement layers, outer edge faces perpendicular to the active region layer, and electrodes by which voltage can be applied across the epitaxial structure and the substrate. A distributed feedback grating is formed on an aluminum free section of the upper confinement layer. The grating is comprised of periodically alternating elements differing from one another in dielectric constant, and thus generally in index of refraction, to provide optical feedback for a selected effective wavelength of light generation from the active region. Because the distributed feedback grating in accordance with the invention is formed in a layer above the active region, regrowth problems and the propagation of dislocations that are encountered with gratings formed below the active region layer are avoided. In addition, it has been found, in accordance with the invention, that by utilizing a confinement layer at least a section of which is aluminum free, the grating may be readily etched in the aluminum free confinement layer to provide a grating surface on which additional epitaxial layers may be grown without difficulty. Such devices are well-suited to being formed to provide a wide emitting aperture, preferably at least 25 xcexcm to provide high power lasing, which may be defined by current confinement.
The invention may be incorporated in semiconductor lasers having a GaAs substrate and epitaxial layers (preferably grown on (100) orientation on the substrate) including an active region layer with single or multiple quantum wells of InGaAs surrounded by InGaAsP barrier layers, optical confinement layers of InGaP, with the distributed feedback grating formed in the top surface of the upper InGaP confinement layer, and cladding layers of InGaAlP or AlGaAs. The thickness of the upper confinement layer and the spacing of the grating from the active region layer is preferably at least about 0.2 xcexcm to ensure small coupling to the grating. Small grating coupling coefficient, xcexa, is needed to maintain a xcexaL product of about unity, where L is the cavity length between the edge faces of the laser, which, in turn, ensures both efficient DFB laser operation as well as single-longitudinal-mode operation to high drive levels above threshold. Since watt-range lasers require long cavities (Lxe2x89xa71 mm), to keep xcexaLxcx9c1 it is of critical importance to have a low xcexa value. Such structures can be formed to operate in the range of 1 watt CW with a linewidth of less than 1 xc3x85 and at 1 watt pulsed (5 xcexcs-wide pulses) with a linewidth of 1.2 xc3x85. Because the upper confinement layer of InGaP is aluminum free, it may be etched in a conventional manner to leave a surface of the grating on which regrowth is readily accomplished.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.